Enter the numbers given in the Sudoku
into the boxes on the left - leave unknown
squares blank.
You can see just the order in which cells can be filled
by clicking "Show Solving Order".
To also display the full solution, click "Show Solution".

If you are solving by hand, you can use the "Save" and "Restart"
buttons to backtrack. Click "Save" to keep one copy of that
state of the puzzle 'on hand'. Click "Restart" to return to the
last saved state.

This applet can also be used to CONSTRUCT Sudoku puzzles.
Add and remove numbers until you arrive at a fully solvable puzzle.

CAVEATS:GIGO: If your input is invalid (e.g., repeat numbers
in rows, columns, boxes, or impossible configuration),
the output will be too.
If you encounter problems with CORRECT input, let me know.

IT'S NOT PERFECT:
The inferences the program draws are limited,
so more difficult puzzles may not completely solved.
In such cases, the user might be able to help,
by making his own inferences and adding some input.
As little as one additional provided
number can turn an unsolved puzzle into a solved one.

Good Source of Puzzles
There are many Sudoku sites on the web.websudoku.com has
an effectively unlimited supply in 4 levels of difficulty.